Heating is one of the basic industrial food processes. Methods of heating includes contact heating (heating sources include fire, sheathed heaters, and ceramic heaters), microwave heating, electromagnetic induction heating and more. Contact heating is a conventional heating method. It requires a longer period of heating time since the temperature of a heat-receiving object rises from its exterior and the heat is gradually transmitted to its interior. Microwave is a kind of electromagnetic wave. Microwaves with specific frequencies enable the polar molecules of the heat-receiving object (for example, water) to resonate, so that the heating begins from the internal of the objects rapidly. When the microwaves start heating the object, however, its intensity distribution thereof will vary according to factors such as the interactions between the microwave power generator, resonant cavity, the heat-receiving object and so on, thereby leading to uneven heating in certain parts.
In order to achieve heating uniformity in microwave heating and for the purpose of continuous production, sterilization and/or pasteurization, many patents employ hot fluids (such as hot water or hot steam) as additional heating mediums (U.S. Pat. No. 7,119,313B2, U.S. Pat. No. 4,962,298). The heat-receiving object is packed inside a sealed container in advance, then sent into a microwave resonant cavity filled with high-pressure fluids. The object in the sealed container is heated by the microwaves and the fluids. As the temperature of the heat-receiving object in the sealed box rises, it would expand due to internal steam or vapor, exerting an outward pressure on the sealed container, therefore causing damage. As a result, the microwave resonant cavity must be pressurized internally so that the pressure inside the microwave resonant cavity counteracts the outward expansion pressure of the sealed container. However, at the initial stage of heating, there is no expansion pressure inside the sealed container, thus excessive pressure in the microwave resonant cavity may cause the unheated sealed container to collapse, while insufficient pressure in the microwave resonant cavity will not be able to resist the outward expansion pressure of the sealed container after it is heated, thereby causing a leakage of the sealed container. Since the maximum heating temperature is in proportion with the exerted pressure, this significantly limits the designing of the heating process. Meanwhile, such a system requires a large-scale microwave resonant cavity and the said microwave resonant cavity must be able to withstand the high pressure of the fluids. Moreover, when the sealed container is fed into and out of the microwave resonant cavity, accurate pressurization and decompression processes are necessary. For this reason, it is impossible for the sealed container to leave the microwave resonant cavity for temperature measurement and monitoring during the process. During heating, the objects are immersed in high-pressure hot water or hot steam and exposed to microwave electromagnetic field, making it difficult to track the temperature and quality of the heat-receiving objects by the usual apparatus and methods. This shows that conventional heating processes lack flexibility. Also such a system easily produces waste heat because of the hot water and hot steam used, which means unnecessary power loss.
One operation prerequisite of contact heating is that the heat-receiving object must be in tight contact with the heat source, so that the heat can be transmitted to the object efficiently. Meanwhile microwave heating requires a suitable resonant cavity to accommodate the heat-receiving object during heating. Although many patents have combined the applications of the two heating methods (U.S. Pat. Nos. 4,900,884, 6,864,468 B2, 5,548,101, 5,177,333) mentioned above, the cavities in the applications thereof are all fixed structures, wherein the heat-receiving object is merely placed in a heating plate or pan, without any means of keeping the object in tight contact with the heating source.
There are many published techniques applying microwaves to continuous heating systems (US2009/0230124 A1, US2012/0103976 A1). Such systems propagates microwaves into several heating spaces while the heat-receiving objects are continuously delivered into and out of the heating system. However, the cavities in the applications are fixed, the object is not sealed in advance and no conductive heating is introduced therein.
In summary, the existing microwave heating systems have the following technical disadvantages:
1. The cavities are fixed structures that cannot be adjusted modularly.
2. Without the use of additional heating mediums, the sealed container is not in contact with the conductive heating source, thereby leading to a lower heating efficiency.
3. When additional heating mediums are employed, the cavities need to be pressurized as well. However, the scale of this applied pressure, the heating temperature and the pressure-resisting strength of the sealed container are mutually dependent, thereby narrowing the processing condition window down to only a few choices.
4. With a conventional system, it is difficult to measure the temperature changes of the heat-receiving object during microwave heating, which means the heating program cannot implement close-loop control. As a result, such systems would not be able to satisfy the requirements of thermal processing of food.